In turbomachinery, it may be important to measure the speed of a driveshaft for the power compressor and the turbine with a high degree of accuracy. Due to the high temperatures, pressures and vibration of such machinery, the most ideal positioning of a driveshaft speed sensor is on the end of the driveshaft forward of the compressor impeller.
A shaft speed sensor assembly generally comprises a shaft speed exciter rotor mounted on the driveshaft and at least one shaft speed magnetic pickup or sensor proximate the exciter rotor. The exciter rotor generally comprises a cylindrical ferromagnetic member that has multiple teeth about its periphery to cause a change in the reluctance of the magnetic circuit for the magnetic sensor as the shaft rotates. Such an exciter may have excessive mass and volume for placement on the driveshaft forward of the compressor impeller in many applications.
Another type of shaft speed sensor for the shaft of an adjustable speed drive has used a cup member serving as an exciter rotor. The cup member retains one end of a compression spring that rotates along with an inner output shaft. The cup member has multiple perforations about its periphery that cause the reluctance of the magnetic circuit of a proximate magnetic pickup or sensor to change as the cup member and output shaft rotates.